In oil and gas wells, it frequently becomes necessary to stimulate product flow to attain economically feasible production rates. For example, a new well may have unacceptable production due to low permeability of the formation or damage around the wellbore. The technique frequently used to stimulate such a well is to pump a fluid into the well until the pressure increases to a level sufficient to fracture the formation which results in the creation of cracks in the formation. These cracks are capable of carrying product to the well at a significantly higher flow rate. This technique is called fracturing or hydraulic fracturing and has been practiced for about forty years.
It is also known that after the formation has been fractured, it is necessary to prop open the newly formed cracks to facilitate the continued flow of gas or oil. Otherwise, the cracks would close under the influence of overburden pressure. The technique used to prop the cracks open is to pump a slurry of fluid and solid particulate material into the fractured formation. The particulate material used can be of varying size and shape. However, it is known that spherical particles provide better permeability than irregularly shaped particles and that closely sized particles, e.g., 25-30 mesh, give better permeability than a wide range of particle sizes. The particles can consist of a variety of materials, such as sand, glass, alumina, bauxite or other materials. Whatever its structure or composition, the particulate material, called proppant or propping agent, must have sufficient strength to withstand the overburden pressure (closure stress) exerted upon the fracture. A common failure of a weak proppant is to disintegrate into fine particles which can clog the cracks created in the fracturing process. Since overburden pressure varies directly with depth, proppants used in deeper fractures are necessarily stronger than those which are suitable for shallower applications.
For depths up to about 5,000 feet, sand, which has a generally accepted specific gravity of 2.62, can be a satisfactory proppant, but its performance degenerates rapidly as depth increases. Sand has long been the most common proppant because of its price and availability. However, man-made proppants have become increasingly more popular, especially in deeper applications where the permeability of sand is inadequate. For comparison purposes, sand is an example of a low density/low strength proppant.
Sintered bauxite or high grade alumina is strong enough to maintain permeability even at well depths greater than 20,000 feet, but these high strength proppants have much higher densities, i.e., specific gravities above 3.6, and require high viscosity pumping fluids or high pumping rates. These high strength proppants require larger pumping equipment and cause accelerated wear rates in fluid carrying equipment. In addition, the high strength proppant is generally higher priced because of higher raw material costs. In addition, it is traditionally priced per unit weight, resulting in higher cost for a given volume of high density proppant. Sintered bauxite is an example of a high density/high strength proppant.
Because of the disadvantages associated with a high density proppant, the goal of the proppant industry has been to create a lower density proppant without sacrificing strength. Lower density proppants, e.g., specific gravity less than about 3.4, have been found to have sufficient strength to provide adequate permeability at intermediate depths and pressures, i.e., 7,000 to 14,000 feet (5,000-10,000 psi). U.S. Pat. No. 4,412,068 which issued to Fitzgibbon on Jan. 24, 1984 describes such an intermediate density/intermediate strength proppant. U.S. Pat. No. 4,522,731 which issued to Lunghofer on Jun. 11, 1985 describes a proppant having a specific gravity less than 3.0 and an alumina content of from 40% to 60%. This low density/intermediate strength proppant of Lunghofer also works well in the intermediate depth range of 7,000 to 14,000 feet. However, the raw materials used to make all these intermediate proppants still represent a significant part of the cost of manufacture primarily because of the alumina content of the ore. Additionally, since most fracturing operations are in the low depth range where sand is used, it is desirable to provide a lightweight proppant which can be produced from inexpensive, low alumina content, raw materials and which exhibits conductivity greater than sand when used in low depth applications.